The events involved in the economy of norepinephrine at the synapse between postganglionic sympathetic nerves and the smooth muscle cells of small pulmonary arteries of dogs are to be determined under normoxic conditions. Subsequently studies will determine whether synaptic events in these vessels change under the influence of acute or chronic hypoxia. Specifically whether hypoxic conditions alters synthesis, retention, release, reuptake and enzymatic degradation of the adrenergic neurotransmitter norepinephrine. The impact on the pulmonary artery of known modulators of norepinephrine function (activation of presynaptic alpha and beta receptors, serotonin, angiotensin II and histamine) will be examined during low and high oxygen tensions in isolated pulmonary arterial strips under basal conditions and during electrical stimulation.